Method of electroless metal deposition

ABSTRACT

Method of electrolessly plating a body with a metal comprising placing the body in a work holder, depositing a film of a substance which catalyzes the deposition of the metal, on the surface to be plated, rinsing the body with a solution of a substance capable of chelating ions of the catalyst and then plating the surface electrolessly with the metal.

United 1 States Patent [191 Arnold Dec. 31, 1974 METHOD OF ELECTROLESSMETAL DEPOSITION Primary ExaminerJohn D. Welsh Attorney, Agent, orFirm-G. H. Bruestle; W. S. Hill;

[75] Inventor. gnjthony Francis Arnold, Rmgoes, P. l van Tricht [73]Assignee: RCA Corporation, New York, NY.

[22] Filed: Apr. 30, 1973 [57] ABSTRACT [21] Appl' 3557l9 Method ofelectrolessly plating a body with a metal comprising placing the body ina work holder, deposit- [52] US. Cl. 117/212, 117/227, 1l7/ 130 E, ing afilm of a substance which catalyzes the deposil17/2l3 tion of the metal,on the surface to be plated, rinsing [51] Int. Cl. B44d 1/18, B32b15/100 the body with a solution of a substance capable of [58] Field ofSearch 1 17/201, 212, 130 E, 227, chelating ions of the catalyst andthen plating the sur- 117/213 face electrolessly with the metal.

[ References Cited 7 Claims, N0 Drawings FOREIGN PATENTS OR APPLICATIONSl,207,63l lO/l970 Great Britain METHOD OF ELECTROLESS METAL DEPOSITIONBACKGROUND OF THE INVENTION When a metal such as nickel is electrolesslydeposited on a surface which is not normally a catalyst for suchdeposition, the surface must first be activated by treating it with acatalyst. Although a number of metals catalyze the electrolessdeposition of nickel, the one most often used is palladium. A very thin,discontinuous film of palladium particles is deposited, as a catalyzingmedium, from a solution of a palladium salt.

In the manufacture of silicon semiconductor devices, for example,solderable electrodes are made by electrolessly depositing nickel on theelectrode areas. It is customary to plate 40 or 50 silicon crystalslices simultaneously in a plastic basket. Some of the palladium saltsolution becomes trapped in cracks and pores in the plastic basket andthis causes subsequent deposition of nickel on the plastic surfaces.Since nickel is a catalyst for the electroless deposition of nickel, theunwanted nickel on the plastic basket can cause instability in thenickel plating bath.

One previous attempt to solve the problem of instability of the nickelplating bath was the addition of an ammonium salt to the nickel platingbath. Ammonium ions complex palladium ions and should therefore neutralize the effect of palladium ions dragged over into the nickel bathfrom the activation bath. When the palladium ions are complexed they canno longer be reduced to metallic palladium which acts as a catalyst forthe deposition of nickel. However, this proved not to be a completesolution to the problem. Nickel still deposited on the plating basketand on the walls of the plating bath container.

The present invention resides in the discovery of the principle that ifa substance which chelates the metal ion of the catalyst being used, isincluded in the rinse between the catalyst deposition and the metalplating, unwanted metal plating on the plating basket (or other workholder) and on the walls of the bath container, is almost eliminated.

DESCRIPTION OF PREFERRED EMBODIMENT An example of the improved processof the present invention, applied to the nickel plating of siliconsemiconductor devices, is as follows.

The objects being plated are crystal slices of semiconducting silicon inwhich a multiplicity of devices such as transistors are beingfabricated. Monolithic type integrated circuits can also be made in thisway. Each slice has a layer of silicon dioxide covering one surfaceexcept where ohmic contacts are to be made to electrode areas. Openingsthrough the silicon dioxide over the electrode areas are assumed to havepreviously been made using conventional photoresist, exposure anddeveloping techniques followed by etching away the unwanted areas ofsilicon dioxide. The remaining hardened photoresist has then beenremoved.

In preparing the slices to be nickel plated on the electrode contactareas, the silicon slices are etched lightly with a solution such as thefollowing:

950 ml/liter The slices are immersed in the solution for only 20 secondsat room temperature (25 C). This etch does not remove much of thesilicon dioxide protective coating, since the coating has been madethick enough to withstand light etching treatments. In treating theslices in this and subsequent steps, about 40 or 50 slices at a time areheld in a basket made of a plastic such as polypropylene. Inert workholders made of other plastics or of other materials such as glass, canalso be used. The surfaces of the work holder must be of a material thatdoes not catalyze the electroless deposition of the metal beingdeposited.

The slices are then treated with a solution of a salt of a metal whichis a catalyst for the deposition of nickel. An example of such asolution is:

Glacial acetic acid 948 ml/liter 48% HF 50 ml/liter 5% by wt. PdCl 2ml/liter The amount of PdCl solution may be varied between about 0.5 and10 ml/liter. If too much PdCl is present, the deposit of palladium metaltends to become non-uniform and this leads to non-uniformity in thenickel deposit.

The amount of 48% HF can be varied between about 20 and ml/liter. Theglacial acetic acid and the hydrofluoric acid improve the wetting of thesilicon surface by the palladium solution and thus permit more uniformdeposition of palladium at low concentration. This saves palladium.

The slices are immersed in the above catalyst solution for 20 seconds at25 C. The time can be varied between about 5 seconds and 60 seconds. Thesolution is reduced by the silicon, and palladium deposits mostly on thesilicon and not on the silicon dioxide. However, some palladium doesoccasionally deposit on the oxide. Palladium also does not normallydeposit on the plastic basket but some palladium solution usuallybecomes trapped in pores and cracks of the basket and this palladiumcannot later be removed by rinsing.

Next, the slices are rinsed with a solution which contains an ammoniumsalt and, preferably, a silicon wetting agent. An example of thissolution is:

2 propanol 200 ml ammonium acetate 20 g deionized water 800 ml Theslices are immersed in this solution for 25 seconds at 25 C. Immersiontime may be between about 10 and 45 seconds. The propanol is used as awetting agent. Other wetting agents, such as other low molecular weightalcohols (i.e., up to C-,), may be substituted. Actually, highermolecular weight alcohols than C alcohols can be used, less preferably,and as the molecular weight increases, the amount used decreases. Theammonium acetate may be used in any amount greater than about 20 g/literup to saturation. Other ammonium salts such as ammonium citrate may alsobe used. Chelating agents other than ammonium salts may also be used.The main criterion is that the amount of chelating agent necessary tocomplex the palladium must not be so great that the nickel plating rateis slowed down unduly.

This solution complexes any palladium salt that may have been trapped inthe plating basket or on the slice and thus prevents that palladium frombeing reduced to palladium metal which would catalyze the deposition ofnickel on surfaces where it is not wanted. The ammonium ion does notcomplex the palladium metal deposited on the silicon slices.

After treatment with the rinse solution, the slices are immersed in anickel plating bath which may have a composition such as the following,per liter of solution. The solvent used is deionized water.

Potassium glycolate 78 g Nickel sulfate 36 g Ammonium acetate g Sodiumhypophosphite g Sulfuric acid to pH 4.8 4.9

The slices are immersed in this solution for 3 minutes at 80 C. Theglycolate and the acetate are chelating agents. Other chelating agentsmay be used. The ammonium acetate is included to complex any palladiumsalt that may have been dragged over from the palladium catalystsolution despite thorough rinsing. The glycolate complexes the nickel.Any soluble nickel salt may be used and the concentration is notcritical. Any conventional reducing agent for nickel may be used inplace of the hypophosphite. Other examples are amine boranes andhydrozine. The pH range may be either 3.8 5.1 or 7.5 10.5 and the pH maybe adjusted with other acids or alkalies.

If the surface being plated is not one which is capable of reducing thecatalyst ion, a coating of a sensitizing agent, such as tin, must firstbe deposited. This must be done, for example, when the substrate beingplated is glass or a plastic. In this case, since the work holder alsowill be sensitized and this would lead eventually to it being completelycoated with metal, the work holder should be changed between thesensitization step and the activation step.

The method applies to any metal capable of being electrolesslydeposited. Cobalt and copper are examples of metals other than nickel.

The method also applies to any of the catalysts which are conventionalfor depositing these metals. Platinum, rhodium and iridium are otherexamples.

I claim:

1. A method of electrolessly depositing a metal on a surface of a bodycomprising:

supporting said body on an inert work holder made of a material whichdoes not catalyze the electroless deposition of said metal,

treating said surface and said work holder with a solution containing asubstance which catalyzes the electroless deposition of said metal,

rinsing said surface and said work holder with a solution of a substancethat chelates ions of the catalyst, and

treating said surface with a plating bath comprising a salt of saidmetal, a reducing agent for said metal salt, a complexing agent for ionsof said metal, and a pH adjusting agent.

2. A method according to claim 1 in which said catalyst is palladium andthe substance which chelates palladium ions is an ammonium salt.

3. A method according to claim 2 in which said body is silicon and inwhich the silicon body has a coating of silicon dioxide except wheremetal is to be deposited.

4. A method according to claim 3 in which said metal is nickel and thesurface to be nickel plated is first lightly etched prior to treatmentwith the palladium solution.

5. A method according to claim 1 in which said rinsing solution alsocontains a wetting agent.

6. A method according to claim 5 in which the wetting agent is a lowmolecular weight alcohol.

7. A method according to claim 1 in which the surface being coated issilicon and said catalyzing solution also contains hydrofluoric acid andacetic acid.

1. A METHOD OF ELECTROLESSLY DEPOSITING A METAL ON A SURFACE OF A BODYCOMPRISING: SUPPORTING SAID BODY ON AN INERT WORK HOLDER MADE OF AMATERIAL WHICH DOES NOT CATALYZE THE ELECTROLLESS DEPOSITION OF SAIDMETAL, TREATING SAID SURFACE AND SAID WORK HOLDER WITH A SOLUTIONCONTAINING A SUBSTANCE WHICH CATALYZES THE ELECTROLESS DEPOSITION OFSAID METAL, RINSING SAID SURFACE AND SAID WORK HOLDER WITH A SOLUTION OFA SUBSTANCE THAT CHELATES IONS OF THE CATALYST, AND TREATING SAIDSURFACE WITH A PLATING BATH COMPRISING A SALT OF SAID METAL, A REDUCINGAGENT FOR SAID METAL SALT, A COMPLEXING AGENT FOR IONS OF SAID METAL,AND A PH ADJUSTING AGENT.
 2. A method according to claim 1 in which saidcatalyst is palladium and the substance which chelates palladium ions isan ammonium salt.
 3. A method according to claim 2 in which said body issilicon and in which the silicon body has a coating of silicon dioxideexcept where metal is to be deposited.
 4. A method according to claim 3in which said metal is nickel and the surface to be nickel plated isfirst lightly etched prior to treatment with the palladium solution. 5.A method according to claim 1 in which said rinsing solution alsocontains a wetting agent.
 6. A method according to claim 5 in which thewetting agent is a low molecular weight alcohol.
 7. A method accordingto claim 1 in which the surface being coated is silicon and saidcatalyzing solution also contains hydrofluoric acid and acetic acid.